An uninterruptible power supply (UPS) prevents power irregularities, such as outages, surges, spikes, etc., from adversely affecting a load. A UPS typically includes a back-up power source, such as a battery, that is used to supply power when power from a main power source is interrupted.
One example UPS has an “online” configuration and includes a rectifier and an inverter. The rectifier and inverter are connected in series between a main alternating current (AC) source and an AC load. The rectifier performs as an AC-to-direct current (DC) converter and the inverter performs as a DC-to-AC converter.
During normal operation, power is supplied to the AC load from the main AC source through the rectifier and the inverter. This provides regulated and filtered power with minimal irregularities, such as voltage spikes, frequency deviations or phase deviations. In one circuit topology, the rectifier provides power to the inverter and to a battery charger via a DC bus. The rectifier may be a pulse width modulated rectifier. The battery charger charges a backup battery. In another topology, when the backup battery is directly connected to the DC bus, the rectifier may be a phase controlled rectifier and be used to charge the battery backup without use of a separate battery charger.
Continuing from the same example, when power from the main AC source is interrupted, power is supplied from the battery to the rectifier instead of from the main AC source. The rectifier performs as a DC-to-DC boost converter and boosts DC power from the battery to the inverter to power the AC load. This allows power to be maintained on the AC load without interruption. When the battery backup is directly connected to the DC bus and when the power from the main AC source is interrupted, power is supplied directly from the battery to the DC bus and not through the rectifier. The described UPS provides uninterruptible clean AC power and DC power. The voltage provided to the AC load is regulated. The voltage provided to the DC bus by the battery backup is not regulated and decreases as energy is drawn from the battery.
Another UPS has an “offline” configuration and includes relays that are coupled in series between a main AC source and an AC load. Power is provided through the relays to the AC load. Power is also provided from the relays to a DC load via a transformer and an AC-to-DC rectifier.
During normal operation, power is provided to the AC load through the relays and to the DC load through the relays, the transformer and the rectifier. Power is also supplied from the transformer to a battery charger to charge a battery. When power from the main AC source is interrupted, the relays are enabled and power is provided to the transformer from the battery via an inverter instead of from the main AC source. Power from the transformer is supplied to the relays to power the AC load and to the rectifier to power the DC load.
Although the offline configured UPS provides both AC and DC power outputs, power supplied to the AC load is primarily supplied directly from an unfiltered and regulated utility power source.